Link using g++

This ensures some C++-specific symbols, such as
`std::_throw_bad_function_call` and probably others, are automatically
included in the link, preventing linker errors when using some standard
c++ library bits (such as `std::function`).

This follows the same change done for SAMD:

    arduino/ArduinoCore-samd#276

Author: matthijskooijman

platform.txt

@@ -20,7 +20,7 @@ compiler.warning_flags.all=-Wall -Wextra
 compiler.path={runtime.tools.arm-none-eabi-gcc-4.8.3-2014q1.path}/bin/
 compiler.c.cmd=arm-none-eabi-gcc
 compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf -MMD
-compiler.c.elf.cmd=arm-none-eabi-gcc
+compiler.c.elf.cmd=arm-none-eabi-g++
 compiler.c.elf.flags=-Os -Wl,--gc-sections
 compiler.S.cmd=arm-none-eabi-gcc
 compiler.S.flags=-c -g -x assembler-with-cpp -MMD